It's quite common for yarrow to change color. This is because the natural color is white, and a variety of hybridizing and genetic manipulation techniques are required to produce colors. With time and/or stress, the plant reverts to its natural white. Stress would probably include too much water, or too much fertilizer. This article covers general yarrow needs: https://www.gardeningknowhow.com/edible/herbs/yarrow/growing-yarrow.htm